1. Field of the Invention
The present invention relates to tuners for radio frequency signal receivers such as televisions and, more-particularly, to a radio frequency tuner with an input filter having electrically adjustable frequency characteristics base on antenna input characteristics.
2. Background Information
Most, if not all, radio frequency (RF) signal receivers, such as radios, televisions, television signal receivers, and the like, include a tuner for selecting a particular radio frequency from the spectrum of available radio frequencies that are input to the RF signal receiver. The RF signal is supplied to the receiver by an antenna and/or cabling.
Tuner RF input circuits for RF signal receivers, particularly television signal receivers, are typically designed based on a known, good 75 ohms source such as provided by cable television systems in order to provide a proper expected performance. Many types of television antennas, particularly those of the indoor variety such as what are known as “rabbit ears”, are not capable of providing a good 75 ohm source. As such, a television antenna (and associated cabling) may present the tuner with an input impedance that is other than 75 ohms, changes through the frequency band of interest, and which contains a resistive component and a reactive component.
The reactive portion of the input impedance will be absorbed by the section of the RF input circuit of the tuner. The resistive component will, in most cases, cause the input transforming network at the RF filter input to create the incorrect bandwidth base on the loaded Q of the network. The unknown impedance may cause the RF input circuit's center frequency to shift, resulting in a mistuning of the center frequency of the filter. Moreover, the effect of the antenna and associated cabling on the RF tuner input may be to adjust the bandwidth of the RF input circuit due to incorrect resistance in the tuned circuit. Additionally, different input circuits may exhibit an alternate effect, whereas the input reactance causes a change in bandwidth, center frequency, or a combination of both may be present. The result in tuner performance will be lower gain, higher noise figure, poor frequency response, and poor adjacent channel performance. The unknown impedance may also cause other tuning problems.
In RF signal receivers such as televisions, it is known to provide a tuner employing electronic alignment. These tuners use an alignment system that allows for an adjustment range on the RF tuning voltage centered around an oscillator control voltage. Basically, these tuners use voltage controlled RF filters for providing adjustability. When an antenna and cabling are used with respect to such electronic alignment tuners, the effect thereof may be to adjust the tuning of the filter(s) off the desired center frequency and/or bandpass frequency characteristics thereof. This “mistuning” can be seen in varying degrees and in varying modes across the entire input frequency range, and additionally varies in response to the antenna, cable, and type of input filter circuit employed.
It is evident from the above that what is needed is a manner of adjusting for an unknown impedance at the input of an RF tuner of an RF signal receiver.
It is further evident from the above that what is needed is a manner of adjustment of frequency response of an input of an RF tuner of an RF signal receiver based on impedance presented by an RF antenna system through its effects on RF signal reception.
It is yet further evident from the above that what is needed is a manner of dynamic adjustment of frequency response of an input of an RF tuner of an RF signal receiver based on impedance presented by an RF antenna system through its effects on RF signal reception.